I hate this question. How can you possibly determine the market size of a startup when the biggest companies (paypal, apple, google, amazon, etc.) have always CREATED a market?
The market size of a startup is the entrepreneur’s ambition. kindle wasn’t on amazon’s original pitch, iPhone wasn’t on apple’s either, maps wasn’t on Google’s plan and so on..
The only way then to make a successful investment is to gauge whether the entrepreneur is determined to run her growing company for the long term and bet on the importance of the product in the future landscape – that startup is golden to invest in.
The traditional rule of figuring if the “market size” is large and taking x% share gives a $B return doesn’t necessarily create big wins.